1. Field of the Invention
This invention relates to top nozzles for nuclear fuel assemblies which accommodate for differences in thermal expansion and irradiation growth of the fuel assemblies and other reactor components and, in particular, to a retrofit expandable top nozzle for use in reactors previously having components composed of essentially the same materials.
2. Related Art
In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies, each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless steel control rod guide thimbles extend above and below the ends of the fuel rods and are attached to the top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles.
The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several functions. First, they position the rod control cluster assembly (RCCA) relative to the guide tubes within the core so that the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods, which are inserted into the fuel assembly as a group or cluster.
The Soviet nozzle also dampens the velocity of the control rods using springs to remove energy when the RCCA rods are dropped into the reactor core during an emergency shutdown, commonly known as a xe2x80x9cscramxe2x80x9d. The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle.
Thus, the Soviet nozzle is designed to function in two positions, free and compressed. As stainless steel is used for the thimbles of the Soviet fuel assembly, the relative separation between the interior of the reactor vessel and the fuel assemblies remains constant once the assembly is in position. Spring loads are such that the nozzles can support the internals, and the spring loads as well as the RCCA positions are fixed so that all functions are static. As a result, the nozzle has built-in references around which the internals are designed. The stainless steel thimbles used in the Soviet design impose higher reactivity cost on the fuel assemblies due to their neutron absorption rate, and they are more difficult to attach to the grids of the fuel assemblies. Non-Soviet fuel assemblies utilize zircalloy for the thimbles which imposes less reactivity cost. However, zircalloy has a different constant of thermal expansion than the stainless steel reactor vessel, and grows during irradiation. Expandable top nozzles, which accommodate for these variations in the dimensions of the different components within the reactor are disclosed in, for example, U.S. Pat. Nos. 4,534,933; 4,687,619; 4,702,882 and 4,986,959. Such nozzles, however, are used in reactors in which the top core plate rests on a core support in the form of a circumferential ledge within the reactor vessel. In the Soviet-type reactor, the core plate rests on and is supported by the top nozzles.
As mentioned, the Soviet design top nozzle is permanently attached to the thimble tubes of the fuel assembly. The above-mentioned patents disclose removable top nozzles and U.S. Pat. No. 5,479,464 took that technology to another step in applying the removable top nozzles to the Soviet-type reactor nozzle design. However, the substitution of zircalloy for stainless steel in some of the fuel assembly components, such as the thimble tubes in which the control rods move, requires further modifications to assure that impact loads experienced by the assemblies can be absorbed without damaging the assemblies or other core components. For example, in the VVER 1000-type Soviet designed reactor, when the control rods scram, they freefall and impact the top nozzle at a very high velocity. This fuel design does not use a dashpot or any other hydraulic mechanical device to minimize these high impacts. In the design described in U.S. Pat. No. 5,479,464, springs are employed to absorb some of this load. However, further means are desired to absorb the shock of the load as well as the load itself. During a scram in a VVER 1000-type Soviet designed reactor, the control rod assembly and its driveline freefall into the fuel assembly. In a standard western fuel assembly design, approximately two feet before full insertion of the control rods into the fuel assembly, the tips of the control rods enter a small diameter portion of the thimble tube called the dashpot. This dashpot is approximately one (1) millimeter larger than the control rods. Because the control rods are moving very fast at this point in the scram, there is a large volume of water which has to be accelerated up past the falling control rods to make room for them in the dashpot. This process causes the control rods to decelerate rapidly, thus lessening the impact velocity of the control rod assembly at the top nozzle adapter plate. The standard VVER 1000 style fuel assemblies do not have a dashpot and therefore the control rod assembly impacts the top nozzle at a much higher velocity. As the kinetic energy is equal to the massxc3x97the velocity2, if the velocity at impact on the VVER 1000 fuel design is four times that of the standard western pressurized water reactor design, then the total energy which has to be absorbed after impact is sixteen (16) times as much.
Accordingly, a new high energy absorption top nozzle is desired that will assure that the impact loads expected during scram events will be absorbed without damaging the nozzle, fuel assembly and/or control rod assembly.
Furthermore, there is a need for an expandable high-energy absorption top nozzle that can accommodate expansion and growth of the zircalloy components of the fuel assembly while supporting the upper core plate in a fixed position.
In addition, there is a need for such an expandable high-energy absorption top nozzle that can absorb the impact of a control rod scram while continuing to support the upper core plate substantially in its fixed location.
These and other needs are satisfied by the invention which is directed to an expandable top nozzle for a nuclear fuel assembly which includes a tubular barrel having a first end on which the upper core plate seats, and a second end. The tubular barrel further includes a hold-down plate circumferentially affixed to the interior wall of the tubular barrel intermediate the first and second ends and substantially spanning the central opening within the tubular barrel. The hold-down plate has a central opening through which an upper hub plunger assembly can pass and a plurality of peripheral secondary openings within which support tubes can move. The expandable nozzle further includes a subassembly comprising a tubular hub having a closed end and an open end, the open end being slidably positioned in the second end of the barrel. The subassembly further comprises a rod ejection plate and support tubes rigidly securing the rod ejection plate to the hub in fixed axially aligned space relation. The ejection plate is provided with apertures aligned with the support tubes that are affixed within the apertures.
An upper hub plunger assembly surrounds, and is coupled to, an upper portion of the central tube and extends through the central opening in the hold-down plate. The upper hub plunger assembly includes a reaction plate that substantially spans the cross section of the tubular barrel and has apertures sized and aligned to slidably receive the support tubes. In the reaction plate""s uppermost position adjacent the hold-down plate, the reaction plate substantially covers the adjacent surface of the hold-down plate. Springs bias the upper hub plunger within the central opening of the hold-down plate, the reaction plate against the lower surface of the hold-down plate and the hold-down plate a predetermined distance from the open end of the tubular hub.
Upon a scram, the RCCA impacts the upper hub plunger assembly, driving it in a direction towards the closed end of the hub. The hydraulic attraction between the hold-down plate and the reaction plate, the hydraulic resistance caused by the displacement of water below the reaction plate as the reaction plate moves down and compression of the springs as the upper hub plunger assembly moves toward the closed end of the hub, absorbs a substantial amount of the energy of the RCCA as the control rods approach the lower portions of the thimble tubes within the fuel assembly.
Preferably, the movement of the tubular barrel in an expanded direction away from the hub is restrained at a given distance from the hub to assure the tubular barrel assembly does not move off the tubular hub.
In the preferred embodiment, the springs surround the support tubes and central tube and substantially extend between the closed end of the hub and the reaction plate. Preferably, the springs are centered and spaced from the exterior wall of the corresponding support tube and central tube that it surrounds so that movement of the spring does not damage the tube""s surface. Desirably, some of the springs extend through openings in the reaction plate and rest against the hold-down plate to support the upper core plate in position during a scram. The support tubes extend to the rod ejection plate. The thimble tubes are removably coupled to the rod ejection plate within the fuel assembly. The central tube is adapted to slidably mount within a corresponding instrument tube within the fuel assembly. In this way, an integral assembly is formed with a removable, expandable nozzle capable of absorbing the large impact loads of an RCCA scram.